mutter - noun

a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

a complaint uttered in a low and indistinct tone

mutter - verb

• talk indistinctly; usually in a low voice
• make complaining remarks or noises under one's breath

mutter - thesaurus

croak   gnarl   grumble   grumbling   maunder   mumble   murmur   murmuration   murmuring   mussitate   mussitation   muttering